The present invention relates to a directional coupler with a high degree of directivity and compact size to facilitate assembly of the coupler and installation of the coupler within a communication system.
Description of Related Art
A photograph of a previously known microwave coupler is shown in FIG. 8. The coupler includes a cup-shaped holder with a resistor and a coupling loop disposed on the large-sized opening of the cup. A connector on the back side of the cup is connected to a measurement system or any external circuit. The cup-shaped coupler is disposed in a housing in proximity to a main transmission line. More particularly, the broad opening is disposed to face a main transmission line, and the cup is rotated relative to the transmission line to control the directivity of the coupling and is positioned within the housing closer to or away from the transmission line to control the degree of coupling.
The above tuning process to position the coupler to achieve the target directivity and degree of coupling takes a significant amount of time and effort. Specifically, rotational adjustments for directivity commonly cause variation in the coupling factor and vice versa. Moreover, as the directivity of the coupler is adjusted by rotating the cup, it is possible that during the tuning process, the directivity will be adjusted in the opposite direction to that desired. Due to these difficulties, a coupler can take up to fifteen minutes to tune correctly. A coupler with the above configuration is conventionally mounted in a communications system to take two measurements, 1) to sample an outgoing signal for signal strength, and 2) to sample how much of the transmitted signal is reflected back from a transmit antenna. As each coupler can take up to fifteen minutes to tune correctly, it may take half an hour to adjust couplers for any given transmission channel.
Additionally, due to the number of components that must be hand-assembled, such as the cup, the resistor, coupling wire and connector, the known coupler is also expensive to manufacture, and has a bulky configuration.
The present invention addresses the above deficiencies. The disclosed invention comprises a coplanar waveguide directional coupler that can be used in conjunction with transmission lines with other waveguide geometries such as a coaxial cable, microstrip and stripline. The coplanar waveguide is formed with a coupling transmission line on a printed circuit board. The coupling line is disposed to face the main transmission line at a predetermined distance to achieve the desired coupling and canted at a particular angle relative to longitudinal direction of the main transmission line to achieve the desired directivity. The coupler includes a load resistor and an optional impedance matching resistor.
When designed for a particular application specified by the coupling factor, directivity, and frequency range, the coplanar coupler of the present invention does not require further tuning. This eliminates the effort in including additional tuning mechanisms during manufacture of the coupler. The ability to precisely design the coupler also eliminates tuning during installation. The coupler also has a compact size compared to known coupler arrangements. The present invention further takes into consideration that the coupling and directivity properties of the designed coplanar coupler may deviate slightly from design parameters to due different manufacturing tolerances in constructing the housing for the main transmission line, for example, as well as electrical tolerances for component resistors and components of the RF sub-systems in which the coupler is used. Therefore, as an additional feature of the present invention, the coupling factor may be fine-tuned by adjusting a tuning screw.